[llvm] [llvm-exegesis][AArch64] Check for PAC keys before disabling them (PR #138643)

[Anatoly Trosinenko via llvm-commits]

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@@ -207,18 +216,38 @@ class ExegesisAArch64Target : public ExegesisTarget {
 
     if (isPointerAuth(Opcode)) {
 #if defined(__aarch64__) && defined(__linux__)
+      // For some systems with existing PAC keys set, it is better to
+      // check the existing state of the key before setting it.
+      // For systems without PAC, this is a No-op but with PAC, it is
+      // better to check the existing key state and then disable/enable them
+      // to avoid runtime crashes owing to unsupported prctl opcodes or if the
+      // CPU implements FEAT_PAuth with FEAT_FPAC (in which case this method
+      // would silently return.).
+      // Hence the guard for switching.
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atrosinenko wrote:

I think, now this comment could be updated to summarize the information collected so far. Something like

> If the CPU implements FEAT_FPAC, authentication instructions almost certainly crash when being benchmarked, so disable all the keys by default. On the other hand, disabling the keys at run-time can probably crash llvm-exegesis at some later point, depending on how it was built. For that reason, the user may pass --COMMAND-LINE-OPTION-NAME in case llvm-exegesis crashes or instruction timings are affected.

... and then before the second `prctl` call we could add an extra explanation like

> PR_PAC_* prctl operations return EINVAL when Pointer Authentication is not available, but no more errors are expected if we got here.

https://github.com/llvm/llvm-project/pull/138643